“We selected 51 types of flavored e-cigarettes sold by leading e-cigarette brands and flavors we deemed were appealing to youth,” researcher Joseph Allen and colleagues wrote in their report, published in the journal Environmental Health Perspectives.

“At least one flavoring chemical was detected in 47 of 51 unique flavors tested. Diacetyl was detected above the laboratory limit of detection 39 of the 51 flavors tested,” they added.

They also found chemicals called 2,3-pentanedione, also known as acetylpropionyl, and acetoin in many of the flavorings. The National Institute for Occupational Safety and Health has found 2,3 pentanedione can cause lung and brain damage in rats.

Diacetyl, a component of artificial butter flavoring, can cause a condition known as lymphocytic bronchiolitis, which in turn can lead to obliterative bronchiolitis — or "popcorn lung".

It’s a rare but debilitating disease that causes vague symptoms such as cough and shortness of breath, and steadily worsens, according to the U.S. Centers for Disease Control and Prevention.

Acetoin is also used in cigarettes and it’s also linked with “popcorn lung”.

The four biggest makers of microwave popcorn have removed the chemicals from butter flavorings after the discovery that it could be causing lung problems in workers, and in at least one consumer successfully sued, saying he developed the condition after eating microwave popcorn daily.

So it’s troubling to see it in e-cigarettes, says Dr. Gina Lundberg, a heart specialist at Emory University and a spokesperson for the American Heart Association.

“Bronchiolitis obliterans is a horrible disease and nobody wants it,” said Lundberg, who was not involved in the study.

“I really think it’s important to find out whether it’s safe,” said Lundberg, who added that she regularly debates with a close relative who vapes and who defends it as a safe alternative to smoking.

“I have friends who are nurses and doctors that are doing it, and there is really this perception that it’s fine.”

“I am surprised how many people in the 45 to 55 category are vaping,” Lundberg added. “I have friends who are nurses and doctors that are doing it, and there is really this perception that it’s fine.”

Lundberg agrees that e-cigarettes can help people stop smoking, but she says many of her patients continue to use e-cigarettes after kicking the habit, and often use flavors that contain nicotine, the addictive and dangerous chemical found in combustible tobacco.

She says vapers should check the labels carefully. But the Harvard team says the labels do not always disclose everything that’s in the product.

“Two companies explicitly stated that their products do not contain diacetyl in written communication, yet in our testing we did find diacetyl in their product,” they said.